Centrifuging process and apparatus



Feb. 10, 1970 w cRAlG ET AL 3,494,542

CENTRIFUGING PROCESS AND APPARATUS Filed May 27, 1968 2 Sheets-Shet 1 SOLIDS DISCHARGE WALTON H.CRA|G, ROBERT J. FINKELSTON, DEAN E. HUMPHREY, ANDRE C. LAVANCHY 0nd LEONARD SHAPIRO BY gm 4.

ATTORNEY.

Feb. 10, 1970 w, cRAlG ET AL CENTRIFUGING PROCESS AND APPARATUS 2 Sheets-Sheet 2 Filed May 27, 1968 v INVENTORS. WALTON H. CRAIG, ROBERT J. FINKELSTO DEAN E. HUMPHREY, ANDRE C. LAVANCHY and LEONARD SHAPIRO B EM 4.

ATTORNEY.

United States Patent CENTRIFUGING PROCESS AND APPARATUS Walton H. Craig, Winnetka, 111., Robert J. Finkelston,

Philadelphia, Dean E. Humphrey, Lansdale, Andre C.

Lavanchy, Devon, and Leonard Shapiro, Upper Darby,

Pa., assignors to Pennwalt Corporation, Philadelphia,

Pa., a corporation of Pennsylvania Filed May 27, 1968, Ser. No. 732,207 Int. Cl. B041) 5/12, 5/06, 5/10 US. Cl. 2337 16 Claims ABSTRACT GF THE DISCLOSURE Solids and liquid are separated in a centrifuge having a helical conveyor coaxially mounted within an elongated bowl tapered at one end, by first feeding a solids-liquid slurry to the bowl, and selectively driving the conveyor and the bowl synchronously during and after feeding to promote sedimentation of the solids without agitation by the conveyor, and then effecting a differential speed between the bowl and the conveyor so as to move the settled solids toward solids discharge means. Optionally, some residual clarified liquid may be removed by a skimming device prior to discharging solids. In addition, an electric drive or brake can be employed to provide an infinite adjustment of the differential speed within a given range.

This invention relates to a method and apparatus for separating solids from liquid-solid mixtures, utilizing a centrifuge having a coaxially mounted screw conveyor within the centrifuge bowl. The invention is especially applicable when the feed mixture is difficult to separate by centrifugation, for example when the solids and the somewhat lighter liquid have nearly the same specific gravity.

Various other factors may contribute to the difficulty of separating some solids from a solids-liquid mixture in a centrifuge of the general type, as shown in Reed Patent No. 3,148,145, wherein a solid bowl centrifuge with an axial screw conveyor is adapted to move solids toward one end of the bowl for discharge. As examples of such other factors, the solids may be gelatinous or tend to agglomerate. Still further, the solids may be of small particle size and become slippery when wet so as to form a soft cake which is difficult for the conveyor to scroll and/ or is susceptible to remixing into suspension with the supernatant liquid when agitated by the screw conveyor.

When there is very little friction between slippery solids and the surface of the screw conveyor, the solids, instead of being conveyed axially and inwardly to the solids discharge port, will move under the influence of centrifugal forces back into the main separating chamber, with the result that the efficiency of separation and the clarity of the effluent are both reduced. In this connection reference is made to US. Patent No. 3,172,851, issued Mar. 9, 1965 to the present assignee in the name of C. M. Ambler, wherein this related problem is discussed in greater detail and a solution is disclosed which is most effective provided lower effluent rates and wetter solids are permissible. The apparatus of C. M. Ambler, however, is to be distinguished from that shown in the cited Reed patent, as well as the prior US. Patents Nos. 2,679,974 and 2,703,676 of F. P. Gooch, in that Ambler provides a solids discharge opening at one end of the centrifuge located radially outward of a liquid discharge opening at the other end of the centrifuge so that settled solids form a relatively im pervious dam extending inwardly of the solids discharge opening, whereby the dam blocks the escape of liquids therethrough toward the solids discharge opening; and

the force of the liquid behind the solids of the dam assists in the discharge of otherwise non-scrollable solids.

It is noted here that past efforts to improve the separation of solids from some solids-liquid mixtures have included the addition of fibrous materials to the sludge-like solids-liquid mixture. Such fibrous materials act as matting agents to provide a stiffer cake formation which is more easily conveyed by the screw conveyor to the solids discharge port. Besides the cost of the fibrous additive, this procedure has a number of disadvantages including the inability to discharge solids which are preferably free of additives. In many instances, however, these disadvantages are justified, especially when the primary purpose of the procedure is to obtain an eflluent with high clarity.

According to the present invention, many of the disadvantages of prior methods and apparatuses are eliminated by a method and apparatus now to be described. The invention proposes a method wherein feed is introduced to a zone of centrifugation for a period of time and then preferably shut off. During feeding, and for a limited period thereafter, the screw conveyor and the centrifuge bowl are operated at synchronous operational speeds in order to promote sedimentation toward the bowl wall of solids having a greater specific gravity than the liquids. Thus, when the screw conveyor turns at the same speed as the bowl there will be no agitation of the solids as they sediment outwardly; and the settling and compaction of the solids along the bowl wall precondition the solids for the scrolling operations to follow. As a compact solids cake develops as an annular formation inwardly of the inner surface of the bowl, a substantially clear body of separated liquid efiluent is positioned inwardly of the cake; and it is therefore optional to skim clarified efiluent from the zone of centrifugation at this time. Alternatively, skimming can be eliminated or it can be performed at other times during the separation procedure. Next, as by energizing an electric brake connected to a gear box, the screw conveyor and the bowl are operated at a differential speed optimum for the given separation conditions and the solids are conveyed to the solids discharge port in relatively dry condition.

This method of operation, consisting basically of separate feed and conveying periods, is then rapidly repeated to maintain a semi-continuous flow of the separated phases.

The various objects, features and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a view, partly in section and partly schematic, of apparatus embodying the invention together with the controls for the operation thereof; and

FIGURE 2 is a fragmentary view of the same incorporating an additional skimmer device as a modification thereof, with like reference numerals being employed to designate correspondingly similar parts.

Referring to FIG. 1 of the drawings, there is illustrated a centrifuge 10 having an imperforate solids bowl 11 disposed within a housing 12 and adapted to be beltalriven by a motor 14 for rotational speeds up to and beyond 3000 times the force of gravity. Suitable bearings 15 are employed at opposite ends of the housing 12 to provide journaled rotational support for the bowl 11. Within the bowl 11 is a helical screw conveyor 16 mounted on a hollow shaft or hub 18, the latter extending in axial direction through the bowl in position to receive an axially extending feed tube 20. The bowl 11 includes a major, or left-hand, portion 21 of cylindrical shape and a minor, or right-hand, portion 22 of conical configuration tapering rightward toward the feed tube 20. In addition, the right-hand end of the bowl 11 is provided with one or more solids discharge ports 24 which are disposed closer to the rotational axis than are liquid discharge ports 26 provided at the opposite, or left-hand end of the bowl. By means of a suitable gear box 30 and braking device 32, connected together by universal coupling 33, the speed of the conveyor 16 relative to the speed of the bowl 11 may be controlled. Such control, as will be described hereinafter in detail is capable of effecting synchronous rotation of the conveyor 16 and the bowl 11. Such control is also capable of effecting a slight difference in speed between the conveyor 16 and the bowl 11, preferably while they rotate in the same direction.

In operation, the feed material or mixture to be separated is delivered through the feed tube 20 and passed into the hollow hub 18 from whence it is discharged radially outwardly through one or more feed ports 34 into a separating chamber 36 interiorly of the bowl 11 and eXteriorly of the shaft 18.

With apparatus of the type described, the left-hand portion 21 of the bowl serves as a continuous solids bowl centrifuge which effects withdrawal of the lighter phase liquid (also termed effluent and supernatant) to the left, as shown, for discharge through the liquid discharge ports 26. When the bowl 11 and the conveyor 16 are operating at different speeds, as aforesaid, the solids are moved axially to the right and also inward toward the rotational axis within the right-hand portion 22 of the separating chamber 36 for eventual discharge through the solids discharge port 24. This conveying action is also referred to as scrolling the solids up the so-called beach defined by the tapered portion 22. It is to be noted that since the liquid discharge ports 26 are disposed radially outwardly of the solids discharge ports 24 the liquid level (also termed pond surface) is such that it intersects with the inner surface of the tapered portion 22 of the bowl at a radial location outwardly of the solids discharge ports 24 and at a radial location equal or inwardly of the outermost portion of the liquid discharge ports 26. Such liquid level, or pond surface, can therefore be seen to define a so-called wet beach for the submerged part of the tapered portion 22, and also a so-called dry beach for the remaining part of the tapered portion 22 disposed inwardly of the liquid level.

By providing a dry beach and eliminating turbulence during the settling phase, in the terms set forth, the separated solids are in drier condition than they otherwise would be; and the present invention is therefore to be distinguished from the Ambler patent previously cited herein.

Gear boxes 30 have long been in use in centrifuge apparatus of the general type described herein; and for the sake of brevity reference is made to US. Patent No. 2,703,676, issued in the name of Fred P. Gooch on Mar. 8, 1955 and assigned to the assignee of the present invention, for a detailed description of a gear box employed for achieving differences in speed of rotation of the conveyor 16 and the bowl 11, e.g. to 60 rpm. It is to be further understood, however, that by holding an extension 38 of a pinion in the planetary system against rotation the conveyor 16 and the bowl 11 will operate at increased differential speed. Similarly when the pinion is rotating in synchronism (zero differential speed), as more and more braking torque is applied for resisting rotation of the pinion extension 38 the greater the differential speed will be. Accordingly, it is the function of the braking device 32 to apply a torque-resisting force to the pinion extension 38 through the universal coupling 33, and the greater the torque resistance the greater will be the speed differential so that with zero slippage the maximum speed differential is obtained.

It is a feature of the present invention that a so-called eddy-current braking device 32 is employed. Such devices are commercially available from the Dynamatic Division of Eaton Yale & Towne, Inc. The braking device 32 comprises a pole ring 40 having an exciter coil 42 surrounding a drum assembly 44, the space therebetween defining an outer parasitic gap. Within the drum assembly 44 is a polarized rotor 46, spaced therefrom to provide an inner working gap through which coolant is circulated. By energization of the exciter coil 42 a magnetic field is produced which generates eddy-currents on the smooth surface of the rotor 46 rotating therethrough. The interconnection between the eddy-current field and the poles of the field assembly produces a braking torque on the rotor 46. The torque, which varies directly with coil current and the speed of the rotor 46, may be infinitely and accurately adjusted by the control system to provide the required braking action..It is an advantage of this kind of braking device 32 that braking torque is developed by an electromagnetic reaction between separated members, thus eliminating wear and the requirement for replacement and adjustment of parts. It can be seen, therefore, that the greater the amount of electrical energy applied to the coil 42 of the device 32, the greater will be the braking action and the greater will be the speed differential between the conveyor 16 and the bowl 11.

Further provided is a solenoid-operated valve 50 in the supply line or feed tube 20. Valve 50 is normally open but when energized stops the flow of feed through the tube 20. In addition, there is further provided a control 52 driven by a timing motor 54. As will be developed more fully, it is the function of the control 52 to effect energization and deenergization of the braking device 32 for controlling differential speed as aforesaid according to a selected program, said program being related to a selected period of time for feeding the mixture to the centrifuge bowl 11. The conveyor 16 and bowl 11 are operated at synchronous speed during feeding and also during any residence period which follows to effect separation of solids from liquid by sedimentation of the former toward the inner surface of the bowl 1]., and then scrolling the solids toward and out the solids discharge port 24 by effecting the differential speed operation of the conveyor 16 and bowl 11 by energization of the braking device 32 for a selected period of time.

By operating the centrifuge apparatus in this sequence, a quiescent condition is created inside the bowl 11 during the first or feed period and the second or residence (cakecompacting) period. This quiescent condition, brought about by synchronizing the conveyor rotational speed with that of the bowl, serves during these periods to prevent disturbances, to soft solids, which otherwise and heretofore were generated by continuous scrolling. During the third or scrolling period of operation, the conveyor 16 is rotated with optimum differential speed for discharging relatively dry solids. It is an advantage of this method that improved clarification takes place along with undisturbed solids compaction as a beneficial effect of producing a deep solids layer during a long residence time in the centrifugal field.

Cyclic operation as described was performed experimentally for the purpose of separating electrolyte liquid from sludge carried therein as a result of electrochemical machining. Such a feed mixture is well known to be difficult to separate by centrifugal methods. The experimental results indicated, however, that the separated solids could be scrolled on a dry beach while giving 2 to 6% solids content in the efiluent, on a volume basis, at average effluent rates up to 1.5 g.p.m. It was also found that quite dry solids were produced even with high differential speeds.

Optionally, the automatic programming of the steps in the process can provide for distinct time periods for each step, or they can overlap one another to some extent. It is preferred, however, that the conveyor 16 be operated at synchronous speed while the feed valve 50 is open, and that synchronous speed be maintained during a cake residence period initiated by shutting off the feed through energization of the solenoid-controlled feed valve 50. At the end of the cake residence period, the feed valve 50 remains closed but the conveyor 16 and the bowl 11 are operated at the desired differential speed to discharge solids, after which the conveyor 16 and the bowl 11 are returned to synchronous speed to await the start of a new cycle by the opening of valve 50.

It is well understood that the elapsed time for each step or period can be adjusted to provide optimum performance and results for the particular feed mixture and process conditions encountered.

MODIFICATION The method and apparatus of the invention is susceptible of further improvement by providing a skimming device 60 capable of discharging efiluent immediately prior to the scrolling operation. As shown in FIG. 2, the skimming device 60 is positioned in the left-hand portion 21 of the bowl 11 adjacent the liquid discharge port 26, and it is further provided with a traversing mechanism 62 capable of moving the skimming device radially inwardly. The dynamic pressure of the rotating liquid coming into contact with the open inlet of the stationary skimming device 60- develops a hydrostatic pressure head which promotes the flow of efiiuent through the passageway 64 of the skimming device 60 and thus reduces the liquid level or pond depth. Obviously the greater the inward movement of the skimming device 60 etfected by the traversing mechanism 62, the lower will be the pond depth. It is an advantage of this feature that solids will be discharged with greater dryness because of the greater length of dry beach. Furthermore, certain solid materials may exhibit better scrollability in a drier state. Moreover, where the solids consist of very fine particles having a tendency of remixing with the liquid While scrolling, there will be less chance for such solids to be discharged with the effluent if a greater percentage of the efliuent is removed prior to scrolling.

The apparatus of FIG. 2 is capable of operating with a relatively deeper pond during the sedimentation part of the cycle, so that there is more room for the collection of solids. And skimming prior to discharging col lected solids increases solids dryness since it is not necessary to discarge solids along a short dry beach, as would be the case if skimming were not performed after eltecting sedimentation with a deep pond.

During operation of the conveyor 16 effluent is preferably prevented from being discharged through the skimmer passageway 64 by retracting the skimming device 60.

In other words, by operating with a deeper pond during the sedimentation part of the cycle there is more room for the collection of solids; and skimming to decrease pond depth prior to discharging solids makes it unnecessary to compromise on solids dryness, that is, accept wetter discharged solids because of a shorter dry beach.

The traversing mechanism 62 can be of the manual type or, as shown, it may be operated by energizing a solenoid-actuated valve 65 to admit pressurized air to a cylinder 66, thus acting against a piston 67 within the cylinder and overcoming spring resistance to transmit motion through a piston rod 68 to the skimming device 60.

The control 52 will now be described in greater detail with reference to FIG. 1, it being understood that the invention can be practiced with other devices, such as those which signal for a completion of the sedimentation phase upon measuring a predetermined thickness of accumulated solids, or a certain degree of liquid, turbidity of the efiiuent discharge as indicated by a reduction in light transmissibility. According to the present invention, the timer 54 unidirectionally turns a shaft 70 on which is mounted a first drum 72 including an arcuate conductive portion 74 (shown cross-hatched) adapted to engage brushes 76, and thus complete a circuit to the coil 42 of the braking device 32 during a portion of one turn of the drum 72. In this coil-energizing circuit is a variable resistor 78 which can be preset to obtain the desired degree of energization, consistent as aforesaid with the amount of braking torque and corresponding speed difierential desired for scrolling. A similar drum 80 is similarly mounted for rotation simultaneously with the drum 72; and it has an arcuate conductive portion 84 (shown cross-hatched) adapted to engage brushes 86 for completing a circuit to energize the solenoid of the valve 50 and thus close valve 50 and prevent the introduction of feed during the residence and scrolling periods.

In the arrangement of FIG. 2, a third drum 87 with an arcuate conductive portion 88 is mounted on shaft 70 and adapted to engage brushes 89 for completing a circuit to energize the solenoid-actuated valve 65 of the traversing mechanism. As mentioned previously, the energization of valve 65 eifects inward traversing of the skimming device 60 whereby the inlet of passageway 64 receives liquid from the bowl 11. The valve 65 can be optionally adjusted to admit air at a selected pressure which exerts a pressure force on the piston 67 sufiicient to overcome the opposing bias of the coil spring within cylinder 66. The extent of inward traverse can thus be preselected in relation to air pressure on the piston and the spring constant of the opposing spring; and when the valve 65 is deenergized pressurized air is released from the cylinder 66 and the spring force returns the skimming device to original outward position.

It is to be understood that, in the broad sense of the invention, synchronous operation which minimizes agitation of the material within the separating chamber 36 embraces minor speed differentials which are substantially less than those normally employed for conveying solids toward the solids discharge ports. Such normal difierential speeds vary, it will be appreciated, with the particular feed and apparatus. Furthermore, it is known that some speed differential may be encountered even when synchronous operation is intended, such as when a resisting torque is imposed on the conveyor 16 by an exchange of momentum between the conveyor hub and the feed material accelerated thereby. In order to compensate for the resisting torque imposed on the conveyor 16, an auxiliary motor 90 may be provided to drive the pinion extension 38 through the rotor 46 and a belt and pulley system generally designated 92, as shown in phantom in FIG. 1.

The additional motor 90 thus may be selectively energized, by applying electric current to leads 94 of motor 90, to drive the pinion shaft extension 38 and ensure positive synchronism between the conveyor 16 and the bowl 11, as when the torque developed between the boWl and the conveyor is so great as to prevent synchronous operation.

Preferably, the motor 90 and the braking device 32 are housed within a single enclosure (not shown).

Although the method and apparatus of the present invention provides for cyclic operation wherein feeding of the mixture to be separated is interrupted during sedimentation or compaction of solids and wherein efliuent is discharged intermittently, it has been found that better results are obtained for mixtures difiicult to separate as described herein than has been obtainable by continously operating similar apparatus.

For example, in one actual test the feed consisted of a 10% concentration of clay solids in a water-clay mixture and the apparatus was operated first continuously and then according to the invention in order to discharge effluent with about 15% of the solids unsedimented, that is, eflluent with 1.5% residual clay solids. On a con tinuous basis the efiiuent discharge rate was 0.4 gallon per minute (g.p.m.), and with cyclic operation according to the invention the mean effluent discharge rate was 0.9 g.p.m., with comparable results.

In a similar test, using like feed material and clay solids concentration, eflluent was continuously discharged at the rate of 0.27 g.p.m. with 5% of the solids unsedimented, or 0.5% residual clay solids. An equivalent rate of 0.78 g.p.m. was obtained in accordance with the invention by feeding the material for 40 seconds at a rate of 1.75 g.p.m., then compacting solids for 10 seconds, and discharging for 40 seconds, with no feed introduced while compacting and discharging, obtaining the same of solids unsedimented or 0.5% residual clay solids.

In the course of the same testing, using the same feed, or continuous operation, with an effluent discharge rate of 0.3 g.p.m., the efiiuent contained 12.8% unsedimented solids, that is, efiluent with 1.28% residual clay solids, with solids concentration (dryness) in the solids discharge of 55.3%. During cyclic operation according to the invention with a mean effluent discharge rate 0f .78 g.p.m., the effluent contained 4.97% unsedimented solids, that is, effluent with 0.497% residual clay solids, and the solids were discharged at 62.4% solids concentration (dryness). Thus, even with a greater mean solids discharge rate the cyclic operation produces drier solids.

It can be seen, therefore, that improved results are obtained with the present invention.

What is claimed is:

1. A centriuge for separating solids and liquid from feed comprising a liquid-solids mixture, said centrifuge comprising:

(a) an elongated hollow bowl mounted for rotation about a longitudinal axis thereof and having at least a portion of its inner surface decreasing in diameter approaching one end of said bowl, said bowl having a solids discharge opening anda liquid discharge opening,

(b) a helical conveyor disposed within the bowl for coaxial rotation either synchronously or at different speeds relative to said bowl and defining therewith a separating chamber, said conveyor extending axially for substantially the entire length of the bowl and having screw flights with distal edges generally complementing the inside contour of the bowl; the conveyor being adapted when turning at a different speed relative to the bowl to move settled solids along said surface portion of decreasing diameter to said solids discharge opening, and said conveyor when rotating synchronously with said bowl permitting un disturbed sedimentation of solids greater in specific gravity than the liquid whereby the accumulation of settled solids on the inside surface of said bowl is promoted,

(c) a feed tube extending along said axis to deliver feed into said bowl,

(d) valve means operatively associated with said feed tube selectively operable for opening or closing said feed tube to the flow of feed therethrough,

(e) and means for selectively driving the bowl and the conveyor either at slightly different speeds sufficient to convey solids to said discharge opening or substantially synchronously.

2. A centrifuge according to claim 1 wherein the liquid discharge opening is disposed a greater distance from the axis of rotation of said bowl than said solids discharge opening.

3. A centrifuge according to claim 2 wherein said liquid discharge opening is spaced from said solids discharge opening along the axis of said bowl.

4. A centrifuge according to claim 1 wherein said liquid discharge opening is positioned at the end of said bowl opposite said one end of said bowl.

5. A centrifuge according to claim 1 wherein the inner surface portion of decreasing diameter is at said one end of said bowl and said solids discharge opening is formed in said surface portion.

6. A centrifuge according to claim 1 further including means for skimming separated liquid from said separating chamber; said skimming means including a movable passageway having an inlet for receiving separated liquid from said separating chamber, an outlet outside said separating chamber, and means for moving the inlet of said passageway to a selected one of various radial positions within said separating chamber.

7. A centrifuge according to claim 1 wherein said selective driving means includes a planetary gear and pinion assembly connecting said bowl and said conveyor for varying the relative speed of said bowl and said conveyor in relation to the braking torque applied to said pinion.

8. A centrifuge according to claim 7 further including means for applying braking torque to said pinion within a range thereof, effecting synchronous rotation of said bowl and said conveyor at one end of said range, and effecting a sufficient difference in speed between said bowl and said conveyor elsewhere within said range to convey separated solids in axial direction to said solids discharge opening.

9. A centrifuge according to claim '8 wherein said torque applying means is an electromagnetic brake comprising a polarized rotor connected to said pinion and disposed within a drum assembly, the latter being surrounded in spaced relationship by a pole ring and an exciter coil, and means for energizing said exciter coil to produce a magnetic field which generates eddy-currents on the rotor, whereby the interconnection between the eddy-current field and said poles produces a braking torque on the rotor which varies directly with the coil current and the speed of the rotor, said energizing means being capable of applying a selected amount of current to said coil for varying the braking torque.

10. Apparatus according to claim 7 further including auxiliary driving means connected to said pinion for applying supplementary rotational torque thereto to ensure operation thereof at a speed synchronous with said bowl.

11. A centrifuge according to claim 1 wherein said selective driving means includes an electromagnetic brake and a planetary gear and pinion assembly, the latter being connected between said brake and said bowl and conveyor assembly for varying the speed differential between said bowl and said conveyor in the range of between 0 and 60 rpm. in direct relation to the current supplied to said brake.

12. A centrifuge according to claim 11 further including a source of electric current, current adjusting means, and a timer control interconnected therewith for initially deenergizing said brake to effect synchronous operation of said bowl and said conveyor and for subsequently energizing said brake with an amount of current preselected with said current adjusting means to effect operation of said bowl and said conveyor at a selected speed differential.

13. A centrifuge according to claim 1 wherein said selective driving means includes an electromagnetic brake for varying the speed differential between said bowl and said conveyor in direct relation to the electric current supplied thereto, and further comprising adjustable means for selectively energizing said brake to effect operation of said bowl and conveyor either synchronously or at differential speed, and electrically actuated valve means for automatically interrupting flow of feed through said feed tube for at least a portion of the time said bowl and conveyor are operating synchronously.

14. Apparatus according to claim 1 further including:

means for reducing the level of liquid in said bowl,

control means responsive to a predetermined level of solids in said bowl for activating said driving means (e) to selectively drive the bowl and conveyor at slightly different speeds for conveying the solids to said discharge opening.

15. A process for separating solids and liquid from a liquid-solids mixture, including the steps of:

(a) feeding the mixture to a separating chamber in a zone of centrifugation having liquid discharge means and a solids discharge opening, the latter being disposed closer to the axial center of said zone than the liquid discharge means,

(b) discontinuing feeding of the mixture,

(c) settling the solids by outward sedimentation thereof in the zone of centrifugation While discharging the separated liquid and while operating a helical conveying surface coaxial with the zone at a speed synchronous with the speed of centrifugation,

(d) engaging the settled solids with the helical conveying surface by operating it at a speed appreciably different than that of the speed of centrifugation and thereby moving the solids with said helical surface axially to said solids discharge opening, and

(e) discharging the separating solids through said solids discharge opening.

1O 16. The process according to claim 15 including the additional step of skimming at least a portion of the separated liquid from said separating chamber prior to said engaging and discharging steps to reduce the level of liquid in said separating chamber.

References Cited UNITED STATES PATENTS 2,703,676 3/1955 Gooch 74-781 X 3,143,504 8/1964 Schneider 2337 3,172,851 3/1965 Ambler 2337 3,428,247 2/1969 Andresen 2337 ROBERT W. JENKINS, Primary Examiner 

